Hydrofoil with lift control by airfreed for watercraft

ABSTRACT

A hydrofoil for watercraft provided with air exit apertures at the under pressure regions of the hydrofoil and extending at least partially over the foil span. Ducts are provided internally of the foil and operatively connected with the apertures to which air can be admitted for the purpose of influencing lift. The front part of the hydrofoil profile possesses an upwardly cambered mean line for producing lift, whereas the rear part of the profile has the maximum profile thickness and a slightly downwardly cambered mean line without intersecting or cutting the chord line for producing approximately equal under pressure at the upper and lower profile sides. The air exit apertures are arranged at the upper and lower profile side at the region of the underpressure produced by the rear part of the profile.

'United States Patent 1191 De Witt HYDROFOIL WITH LIFT CONTROL BY 477,206 10/1915 France ..244/35R AIRFREED FOR WATERCRAFT FOREIGN PATENTS OR APPLICATIONS [75] Inventor: Hermann De Witt, Lucerne, Primary 1 S i l d Assistant ExaminerDona1d W. Underwood [73] Assignee: Supramar Ag, Luceme, Switzerland Anomey Agent or Firm Werner w' Kleeman [22] Filed: July 2, 1973 [57] ABSTRACT [2]] Appl. No.: 375,950

A hydrofoil for watercraft provided with air exit apertures at the under ressure re ions of the h drofoil 52 US. Cl. 114/665 H; 114/126; 244/123; and extending at lfast partiangy Over the g, Span 2 416/90; 416/223; 416/231; 416/242 Ducts are provided internally of the foil and operaii lgzidCkf L tively connected with the apertures to which air can 1 be admitted for the u ose of influencin lift. The 244/77 123;116/223 223 223 front part of the hygro gil profile possess es an up- 231; 303/21 P wardly cambered mean line for producing lift, whereas the rear part of the profile has the maximum profile [56] References cued thickness and a slightly downwardly cambered mean UNITED STATES PATENTS line without intersecting or cutting the chord line for 3,146,751 9/1964 Von Schertel 114/665 H producing pp x y equal under pressure at the 3,156,209 11/1964 Ask 114/665 H upper and lower profile sides. The air exit apertures 3,335,687 8/ 9 Von Schertel 6 H are arranged at the upper and lower profile side at the H region of the underpressure produced the rear part 3,578,819 5/1971 Atkins 303/21 P of the profile.

9/1920 Germany 416/242 sensors 5 control umt integrator 10 8a Iydraulic cylin der US. Patent 0m. 28, 1975 Sheet 1 of 3 3,915,1U6

US. Patent Oct. 28, 1975 Sheet 2 of 3 3,915,106

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control -/7 un/t integrator 10 M 80 hydroulic I (Mr valves cylinder l 73d 5 x 13b Q 13a /12 13c-- US. Patent Oct. 28, 1975 Sheet3of3 3,915,106

V m w Emma HYDROFOIL WITH LllFT CONTROL BY AIRFREED FOR WATERCRAFT BACKGROUND OF THE INVENTION The present invention relates to water vehicles equipped with hydrofoils, the lift of which is controlled through the feed or supply of a variable quantity of air to the low pressure regions of the profile.

This principle of lift control is already known to the art. The air is fed through apertures extending in the span direction at the upper side of the foil profile. With an increased quantity of admitted air the lift force is decreased. Heretofore an average air quantity has been fed during undisturbed cruising of the watercraft in order to obtain both a decrease as well as an increase in lift. By increasing the air quantity beyond the mean lift air quantity, the lift is reduced and by decreasing the air quantity to zero, i.e. the valves completely closed, lift is augmented to its maximum.

This method of lift control has several disadvantages. First of all, the changes in lift which can be attained with this technique are rather small in relation to the mean lift and such are not sufficient for hydrofoils of slow vehicles where the mean lift coefficient is high. Another drawback of this control technique, where already half of the quantity of the air is discharged in smooth water, is the additional drag which occurs also when travelling without the control in operation.

SUMMARY OF THE INVENTION It is a primary object of the present invention to provide an improved construction of hydrofoil with lift control by air feed for watercraft which is not associated with the aforementioned drawbacks of the prior art proposals.

In order to overcome the previously discussed drawbacks, a basic concept of the present invention provides a foil without air supply at the undisturbed cruising speed in order to avoid additional drag. Increase of the lift is obtained by feeding the lower section side of the foil with air and a reduction of lift is obtained by admitting air to the upper side of the foil. Although feeding of the air to the lower side for increasing lift is known as such in the art, the concept of this development could not be heretofore realized because there was not known to the art a foil section which could maintain low pressure at its lower side throughout all occurring angles of attack. However, this is an absolute requirement for the effectiveness of a control. The foil profiles which are normally employed have an upwardly cambered mean line for producing lift and as a consequence thereof low pressure at the upper side (suction side) and overpressure at the lower side (pressure side). Even if a weak suction prevails at the lower side due to a flat mean line of camber, the subpressure will disappear ifthe inflow angle becomes positive. This occurs periodically in a seaway which is under the effect of the orbital motion in the water, which varies the flow angle. Especially in the case of fully submerged hydrofoils, where stabilizing forces are not generated by immerging and emerging foil portions, there is required a lift control which remains effective throughout all flow conditions.

The profile section according to the invention possesses a positive design lift coefficient and nonetheless a strong subpressure not only on the upper side but also on the lower side in the range of about 50 to 70% of the chord length behind the leading edge. As is known, the

influence of the changes of the angle of attack on pressure distribution is rather small at this part of the foil profile, and for which reason the subpressure on both sides is maintained in aseawayunder varying flow angles. The front part of the hydrofoil possesses an upwardly cambered mean line for producing lift, whereas the rear part of the profile has the maximum profile thickness and a slightly downwardly cambered mean line without cutting the chord line for producing approximately equal underpressure at the upper and lower profile side. The air exit apertures are placed at this rear part at the upper and lower profile sides at the region of the underpressure.

The center of pressure of such a profile is situated very close to the aerodynamic center (one quarter chord length behind the leading edge valid for most of the subcavitating profiles) and consequently, location of the center of pressure is nearly independent of the angle of attack. This constitutes a remarkable advantage for profiles with adjustable angle of attack because the pitching moment with respect to a favorably situated turning axis is negligible.

The main advantages of the hydrofoil according to the invention with the air feed or air supply to both sides of the foil profile, over known air-fed hydrofoils, are considerable:

a. According to tests, the attainable lift increase by pressure side feeding is as large as the lift decrease by suction side feeding, and in consequence thereof thecontrollable range of lift is doubled.

b. The additional drag due to the air feed is eliminated as long as the hydrofoil is travelling in smooth water, whereas the additional drag in a seaway is reduced to a minimum, because the positive and negative lift variations always pass the non-fed state with no additional drag, thus reducing the mean value of additional drag.

The ideal balance between lift and load in the nonfed state will be seldom encountered in practice. If the load is too high or the speed too low, then the control signal will open the valve for the air supply to the lower side, whereby lift is increased. If, on the other hand, the lift is too high, then, the control will deliver the opposite signal and open the valve for air feed to the upper foil side. When deviating from the design lift coefficient, the occurring drag is, however, lower when correction is made by change of angle of attack or flap angle than by air feed. Therefore, according to a further embodiment of the invention, it is contemplated that the adjustment of the mean lift coefficient is undertaken in this manner, whereby the parts move with a very low angular speed, thus requiring only a small hydraulic power. The high frequency lift changes in a seaway are accomplished as described by change of the admission of air, and the power requirements for this purpose to actuate the air valves does not even amount to 1% of the corresponding hydraulic power input for moving foil or flaps. By subdividing the control into two frequency ranges with different functions the range of lift changes by the air feed is fully maintained on either foil side and constitutes another advantage of the inven-. tion.

According to the invention, the output of the control unit for the lift changes is employed directly for actuation of the air valves and at the same time fed into an integrator with a large time-constant. The output of this '3 integrator represents the difference between the average and the ideal lift coefficient with respect to air feed. This difference is employed to control the angle of attach or the flap angle until the required means lift coefficient is attained without air feed and the signal from the aforementioned integrator disappears.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood and objects other than those set forth above, will become apparent with consideration is given to the following detailed description thereof. Such description makes reference to DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS At this point it is mentioned that throughout the various embodiments the same reference characters have been generally employed for the same or analogous components. In the embodiments of FIGS. I and 2, there have been depicted by way of example the profile and the system of air feed for two different constructions of hydrofoil. It is to be understood that the'section half of each profile in front of the center line aa possesses a mean line I which is cambered upwardly for the generation of lift. The mean line of the other section half behind this center or middle line aa is slightly downwardly cambered without cutting the chord line, as shown. It merges near the trailing edge into the chord line. In this way the induction from the circulation of the front part of the foil is compensated and no lift is produced at this portion. Consequently, there results and equal low pressure at the upper and lower section sides of the rear part of the foils. Due to the biconvex shape of the rear part of the profile, which possesses the maximum section thickness, it is possible to increase or decrease the lift by air feed or air supply to the pressure or suction side.

The air exit openings, namely the openings 2a, of the embodiment of FIG. 1 and the openings 2a, 2b, 2c and 2d of the embodiment of FIG. 2, are placed at the rear part of the corresponding foil profile at the region of maximum curvature of both sides where the maximum suction is produced. In the embodiment of FIG. I, there are only provided in the span direction of the foil two rows of air-exit apertures, of which the row 2a is located on the upper side of the foil and the other row 2c on the lower side of the foil. In the embodiment of FIG. 2, there are provided four air exit rows, namely the rows of air exit apertures 2a and 212 on the upper side of the foil and the rows of air exit apertures 20 and 2d on the lower side of such foil. It is possible to arrange further rows of air exit openings or apertures, whereby their number may be different on the upper and lower side of the foil. The ducts, namely the ducts 3a, 3c of the embodiment of FIG. 1 and 3a to 3d of the embodiment of FIG. 2, provided in the corresponding hydrofoil, are connected with ducts such as the ducts 13a to 13d (FIG. 3), in the strut 12 to which the hydrofoil is attached. Air is sucked-in from the free atmo sphere through this system of ducts and controlled by valves.

A greater range of increase or decrease of lift changes by air feed can be attained by supplying the foil with compressed air instead of atmospheric air. Be-

cause the applied overpressure is relatively low and the.

air quantity small, the compressed air can be generated by a blower driven by a piston engine. In the case of a gas turbine driven vehicle, the compressed air can be bled or drained from the compressor. Generally, the pressure of this air is too high for air feed of the hydrofoil and must thus be reduced by a suitable pressure reduction valve.

According to the simplest technique proposed by the invention, the two ducts 3b and 3d of the embodiment of FIG. 2 are fed with overpressure. However, the corresponding valves only open if the two rows of ducts 3a and 3c which are arranged further backwards at the foil are nearly saturated by atmospheric air. Another technique is to combine the supply or feeding of air with atmospheric pressure and with overpressure in the same duct or ducts; for example the ducts 3b and 3d. To carry out this function there is required a special valve.

The control system for the described combined control of the angle of attack and air valve has been represented by the block diagram of FIG. 3. The measurement values of the sensors 6, which measure the position and movement of the vehicle, are processed in known manner at the control or regulator 7, as for instance disclosed in US. Pat. No. 3,156,209, granted Nov. 10, 1964 and incorporated herein by reference, the commands of which, for the purpose of changing the lift, are directly transmitted via the conduit 8a to the schematically represented air valves 9' of the ducts or channels 3a to 3d. The same commands are delivered via the branch conduit 8b to an integrator 10.

This integrator 10, owing to its large time-constant, measures the average or means deviation of the foil lift from the non-vented condition. If, for instance the.

venting of the suction side ducts 3a and 3b predominates then the mean foil lift is too large and the integrator l0 delivers to the hydraulic control cylinder 11 a command signifying a slow reduction in lift by upwardly moving the rear or trailing edge 12 of the foil. As a result, and depending upon the construction of the foil, either the flap angle of the flap 4 is reduced in known manner, or if instead of a movable flap, there is provided the pivot point or point of rotation 5 for the entire foil, then the angle of attack of the foil is reduced in known manner.

In any event the upward movement of the rear edge 12 of the foil will continue in known manner for such length of time until the pressure side ducts 3c and 3d are vented as frequently as the suction side ducts 3a and 3b and the command emanating from the integrator 10 has disappeared. The opposite operations occur automatically when, by means of the control unit or control 7, the pressure side ducts are more frequently vented than the suction side ducts.

The branching of the output of the control 7 at the conduit 8a for the relatively high-frequency movement of the air valves 9 and in the conduit 81: via the integrator 10 for the low-frequency actuation of the hydraulic cylinder 11 does not complicate the control 7. In this manner the previously described advantages of the hydrofoil with venting at both sides can be maintained for all operating conditions of the vehicle.

To facilitate take-off of the watercraft, the trailing edge of the foil can be provided with an adjustable trimming flap as is known in this particular art, in the event that the described flap for correction of the nonfed state is not applied.

if, on the other hand, trailing edge flap is employed for correction of the mean lift coefficient, then the foil section with the location of its maximum thickness positioned far backwards along the foil profile is not only advantageous for the flap hinges but also with respect to hydrodynamics. Flow separation and inception of cavitation over the deflected flap are delayed due to the relative weak curvature near the flap axis and the fact that the mean lift generates no lift in the rear part of the hydrofoil. In this way the applicability of a flap is extended into a higher speed range compared with known hydrofoils.

While there is shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims. Accordingly,

What is claimed is:

1. A hydrofoil for watercraft provided with air exit apertures at the under regions of the hydrofoil, said air exit apertures extending at least partly over the span of the foil, ducts provided internally of the foil and connected with said air exit apertures and to which air is admitted for the purpose of influencing lift, said hydrofoil encompassing a hydrofoil profile having a chord line and a from half and a rear half, the front half of the profile of the hydrofoil having an upwardly cambered mean line for producing lift and the rear half of the profile of the hydrofoil possessing the maximum thickness of the profile and a slightly downwardly cambered mean line for producing approximately equal underpressure at the upper and lower sides of the profile, said cambered mean line being at the same level or above the chord line over the length of the chord line, said air exit apertures being located at the upper and lower sides of the profile at the region of the underpressure produced by the rear half of the hydrofoil profile.

2. The hydrofoil for watercraft as defined in claim 1, further including valve means for controlling admission of a quantity of air to the air exit apertures.

3. The hydrofoil for watercraft as defined in claim 1, wherein the height of the camber of the mean line of the front half of the profile and the angle of attack of the hydrofoil are dimensioned such that at the cruising speed of the watercraft and in the absence of admission of air to the upper and lower sides of the hydrofoil the load on the hydrofoil is substantially balanced by the lift of the hydrofoil and including valve means for admitting air via said air exit apertures to the lower side of the hydrofoil to increase lift and to the upper side of the hydrofoil to decrease lift.

4. The hydrofoil for watercraft as defined in claim 3, wherein the hydrofoil has a trailing edge, flap means pivotably connected with the trailing edge of said hydrofoil for controlling the balance of the load on the hydrofoil in the absence of admission of air to the upper and lower sides of the hydrofoil, a control unit for sensing the motion of the watercraft, an integrator with a large time-constant, said. control unit generating output signals for directly actuating said valve means and for causing deflection of said flaps after integration by said integrator. 

1. A hydrofoil for watercraft provided with air exit apertures at the under pressure regions of the hydrofoil, said air exit apertures extending at least partly over the span of the foil, ducts provided internally of the foil and connected with said air exit apertures and to which air is admitted for the purpose of influencing lift, said hydrofoil encompassing a hydrofoil profile having a chord line and a front half and a rear half, the front half of the profile of the hydrofoil having an upwardly cambered mean line for producing lift and the rear half of the profile of the hydrofoil possessing the maximum thickness of the profile and a slightly downwardly cambered mean line for producing approximately equal underpressure at the upper and lower sides of the profile, said cambered mean line being at the same level or above the chord line over the length of the chord line, said air exit apertures being located at the upper and lower sides of the profile at the region of the underpressure produced by the rear half of the hydrofoil profile.
 2. The hydrofoil for watercraft as defined in claim 1, further including valve means for controlling admission of a quantity of air to the air exit apertures.
 3. The hydrofoil for watercraft as defined in claim 1, wherein the height of the camber of the mean line of the front half of the profile and the angle of attack of the hydrofoil are dimensioned such that at the cruising speed of the watercraft and in the absence of admission of air to the upper and lower sides of the hydrofoil the load on the hydrofoil is substantially balanced by the lift of the hydrofoil and including valve means for admitting air via said air exit apertures to the lower side of the hydrofoil to increase lift and to the upper side of the hydrofoil to decrease lift.
 4. The hydrofoil for watercraft as defined in claim 3, wherein the hydrofoil has a trailing edge, flap means pivotably connected with the trailing edge of said hydrofoil for controlling the balance of the load on the hydrofoil in the absence of admission of air to the upper and lower sides of the hydrofoil, a control unit for sensing the motion of the watercraft, an integrator with a large time-constant, said control unit generating output signals for directly actuating said valve means and for causing deflection of said flaps after integration by said integrator. 